1. Field of the Invention
The present invention relates to an electrophotographic multicolor image forming method and apparatus, more particularly, to a multicolor image forming method for forming a plurality of toner images on an image retainer in a superposed manner and an apparatus therefor.
2. Description of the Prior Art
The aforementioned electrophotographic image forming is realized by carrying out two or more cycles each including the steps of (1) charging, (2) image exposure and (3) development on an image retainer which has a photoconductive layer on a conductive substrate (as is disclosed in Japanese Patent Application No. 184381/1983). By using an image retainer having a transparent insulating layer on the outer side of a photoconductive layer, another method carries out two or more cycles each including the steps of (1) primary charging, (2) simultaneous secondary charging and image exposure, (3) uniform charging, and (4) development, or still another carries out two or more cycles each including the steps of (1) primary charging, (2) secondary charging, (3) image exposure and (4) development (as is disclosed in Japanese Patent Application No. 183152/1983). These methods make possible the multicolor developments or image compositions on the image retainer, and these superposed images can be transferred to the transfer material by a single transfer process, to provide an apparatus for forming a multicolor or composed image with a simple structure. Therefore, the developing method has to be carried out under the conditions, as disclosed in Japanese Patent Application No. 57446/1983 or 192712/1985, by using a developer made of a mixture of a non-magnetic toner and a magnetic carrier. This developing method belongs one kind of magnetic brush developing method and is characterized in that only the toner is caused to fly onto the latent image face of the image retainer by an AC bias while the magnetic brush being kept away from contact with the image retainer.
One example of the aforementioned image forming apparatus is practised by a developing device which forms latent images of different colors by latent image forming means and uses toners of colors corresponding to the respective latent images.
This multicolor image forming apparatus is represented by that in which an image retainer (which will be called a "photosensitive member", as the case may be) having a photoconductive substance on a conductive substrate is irradiated with an optical ray such as a laser beam to form electrostatic latent images. In this apparatus, the multicolor image is formed in the manner, as shown in the flow chart of FIG. 11.
FIG. 11 shows the changes of the surface potential of the image retainer, in which: PH designates an exposed portion of the image retainer; DA an unexposed portion of the image retainer; T.sub.1 a toner deposited on the image retainer by a first development; T.sub.2 a toner deposited on the image retainer by a second development; and DUP the rise in the potential caused as a result of deposition of the toner T.sub.1 on the exposed portion PH by the first development. For simplicity of description, the polarity of the latent image is assumed to be positive.
(1) The image retainer is uniformly charged to have a constant positive surface potential E by a charging device;
(2) A first image exposure is given by using a laser, a cathode ray tube or an LED as an exposure light source so that the potential at the exposed portion PH drops in accordance with the amount of light of exposure.
(3) The electrostatic latent image thus formed is developed by a developing device to which is applied a positive bias substantially equal to the surface potential E of the unexposed portion. As a result, the charged positive toner T.sub.1 is deposited on the exposed portion PH at a relatively low potential to form a first toner image. The region formed with this toner image has its potential rising by the DUP as a result of deposition of the charged positive toner T.sub.1 but normally takes a potential different from that of the unexposed portion DA.
(4) Next, the surface of the image retainer formed with the first toner image is subjected to a second charging treatment by the charging device so that an even surface potential E prevails irrespective of the presence or absence of the toner T.sub.1.
(5) The surface of this image retainer is subjected to a second image exposure to form an electrostatic latent image.
(6) This electrostatic latent image is developed with a charged positive toner T.sub.2 of a color different from that of the toner T.sub.1 like the previous step (3) to form a second toner image.
This process is carried out a desired number to form a multicolor toner image on the image retainer. This multicolor toner image is transferred to a transfer material and is fixed with heat or under pressure to form a multicolor recorded image. In this meanwhile, the image retainer has its residual toner or charges cleaned off from the surface thereof so that it may be used for a subsequent multicolor image formation, after its multicolor toner image has been transferred to the transfer material.
These multicolor records thus obtained have strict requirements for the color balance because they will appeal directly to the eyes.
(1) The recording characteristics of gradations of the individual color components should be sufficiently identical.
(2) The aging and individual dispersions in the color balance should be small.
We therefore have proposed a method including the steps of forming a reference toner image on an image retainer, detecting the reflective density of the toner image, and feeding back the reflective density detected to image forming conditions, as is disclosed in Japanese Patent Applications Nos. 158456/1984, 179119/1984 and 188690/1984. However, this method has succeeded in holding the maximum density substantially constant but has failed to obtain a condition for holding constant the gradation expression characteristics which are the most important for color expressions.
On the other hand, Japanese Patent Laid-Open No. 57868/1985 has disclosed a method which includes the steps of transferring a toner image to a transparent member of a transfer device and detecting the density. However, this method is required to have the transfer device to be driven highly accurately in addition to the image retainer, thus raising a problem that the overall structure of the apparatus is complicated and enlarged.